mod_pubsub_serverinfo: Detect existence of pub/sub node Instead of blindly trying to create the pub/sub node to publish items to, a service discovery query is performed to check if node creation is required. Added various bits of warn and debug logging, to give a user better feedback if and why something is failing.

---
labels:
- 'Stage-Alpha'
summary: Implements BOSH pre-bind
...

For why this can be useful, see
https://metajack.im/2009/12/14/fastest-xmpp-sessions-with-http-prebinding/

# To enable this module

Add `"http_prebind"` to `modules_enabled` on an anonymous virtual host.

This only works on anonymous ones for now.